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Abstract:

The Söhlde Formation (Upper Cenomanian-lower Upper Turonian) of Lower Saxony and Sachsen-Anhalt is characterized by an alternation of red and white limestones of a pelagic biosedimentary system, deposited ca. 200 km distant from the nearest coastline on the European Cretaceous shelf sea at a paleolatitude around 45° N. Seven sedimentary cycles of ca. 430 ky duration can be recognized, each of which is separated by discontinuities and/or significant facies changes. White limestones and marl–limestone alternations were deposited mainly in intrashelf depressions and/or during relative sea–level highs. The red limestones were deposited on intrashelf swells above and shortly below storm wave base. Storm-and current-induced advective pore-water flow associated with low accumulation rates in a nutrient-depleted intrashelf swell setting (low Corg flux into the sediment) resulted in an excess of oxygen in the sediment column and an early diagenetic window, in which ferric iron minerals were generated, causing the red pigmentation. The source of the iron was most likely clay minerals, inasmuch as a positive correlation between clay content and red pigmentation is observed. No trace of microbial activity associated with the genesis of the red color can be confirmed yet.

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